Radiation-emitting semiconductor device, method for fabricating same and radiation-emitting optical device
The invention concerns a radiation-emitting semiconductor device in which a multilayer structure comprising a radiation-generating active region has assigned to it a window layer for coupling out radiation.
It relates in particular to a radiation-emitting semiconductor device comprising a nitride-based active multilayer structure arranged on an SiC-based epitaxial growth substrate and to a radiation-emitting optical device equipped with such a radiation-emitting semiconductor device.
The invention is further directed to a method for fabricating the radiation-emitting semiconductor device and to an optical device comprising a semiconductor body of this kind.
Typically, present-day radiation-emitting optical devices, especially light-emitting-diode devices, are equipped substantially exclusively with right-parallelepipedal radiation-emitting semiconductor devices, usually embedded in transparent casting material. A major difficulty with such devices is the large difference between the refractive indices of the semiconductor materials normally used in optical semiconductor devices (n greater than 2.5) and those of the conventionally available casting materials (epoxy resin, for example; nepoxy greater than  greater than 1.5). Thus, the critical angle of total reflection at the boundary between the semiconductor body and the casting material is very small. It is for this reason that a substantial portion of the light generated in the active region fails to be coupled out of the semiconductor body, due to total reflection on the chip surfaces, and goes to waste inside it. Given a defined electrical current flowing through the semiconductor device to generate the light, the brightness of the device is therefore limited.
In the case of GaN-based light-emitting diode chips, where the epitaxial layer sequence is arranged on a substrate (for example, a silicon carbide substrate) that has a higher refractive index than the epitaxial layer sequence, the special problem also arises that with the conventional right-parallelepipedal chip geometry, the portion of the radiation coupled out by the lateral sides of the substrate is output in the direction of the back of the chip at a very acute angle with the lateral side of the substrate. This radiation is therefore incident on a housing mounting surface to which the chip is attached, striking it at a very steep angle and very close to the chip. This entails the disadvantages that, first, because of the acute angle of incidence, a large portion of the radiation is absorbed in the chip mounting surface, and second, there is considerable risk that some of the radiation will strike and be absorbed by the conductive adhesive normally used to secure the chip.
DE 198 07 758 A1 proposes a radiation-emitting semiconductor body which, to increase the light yield of the active region in the intended direction of radiation of the semiconductor body, comprises a subsequently added so-called primary window layer whose continuous lateral face forms an obtuse angle with the plane of extension of the multilayered heterostructure. The continuous lateral face forms an obtuse angle of between 110xc2x0 and 140xc2x0 with the plane of the active region. The primary window layer is formed by the epitaxial growth substrate or by an epitaxial layer grown separately thereon.
In addition, the semiconductor body in accordance with DE 198 07 758 A1 can comprise a further, so-called secondary window layer that is arranged by epitaxy or wafer bonding on the side of the active region facing away from the primary window layer, i.e., on the underside of the semiconductor body, and whose continuous lateral face forms an angle of between 40xc2x0 and 70xc2x0 with the plane of the active region. The semiconductor body therefore has obliquely disposed chip sides that are continuous from the top side to the underside.
This chip geometry serves primarily to make the face of the chip extending parallel to the active region larger than the active region and to cause all of the light striking the oblique side walls of the primary window to be reflected internally in the intended direction of radiation.
In addition, the secondary window layer performs the task of coupling out of the semiconductor body, via the oblique lateral faces of the secondary window layer, light that is emitted by the active region toward the back, i.e., in the direction of the mounting surface of the semiconductor body.
To reduce the outputting of light in the backward direction and to deflect this light to the front side, preferably within the very semiconductor body, a reflective coating is provided on all of the oblique sides of the chip.
This known chip geometry, which is intended primarily to improve the coupling-out of light via the front side, raises the following problems in particular:
(i) In the fabrication of the oblique lateral faces, a substantial portion of the area of the active epitaxial layer sequence present on the wafer is wasted, since the oblique lateral faces are produced by making a V-shaped trough from the active-region side.
(ii) The thickness of the secondary window layer is severely limited in order to preserve a sufficiently large chip mounting surface, thereby ensuring that
no tilting of the chip occurs as it being mounted in a light-emitting-diode housing,
there is current spreading to the entire active region, insofar as possible,
there is adequate heat dissipation from the active region,
the chip has sufficient mechanical stability. Thus, its width is preferably only about 10 to 40% of the lateral width of the active region.
(iii) The oblique sides meet the chip mounting surface of a light-emitting-diode housing to form a wedge-shaped gap, which in the case of conventional plastic LED housings is usually filled with transparent casting material. When the temperature of the device increases during operation and/or due to an increase in ambient temperature, as occurs, for example, in motor-vehicle applications, because of the high thermal expansion of ordinary casting compounds the chip is subjected to considerable mechanical stresses, due to which the risk of delamination of the chip from the chip mounting surface of the housing is substantially increased over that of right parallelepipedal chips.
(iv) The fabrication of the secondary window layer is technically very onerous, since this layer must be added by epitaxial growth or wafer bonding.
(v) The underside of the chip, which is the mounting surface, is the smallest surface of the semiconductor body, over which the widely projecting upper window region is disposed. There is accordingly a high risk that with an automatic chip mounting technique conventionally employed in chip mounting, usually a pick-and-place method, tilting of the chip and hence tilting of the beam axis of the light-emitting-diode device may occur. This risk is reduced if only one primary window layer and no secondary window layer is present.
(vi) The thickness of the lower window layer, if any, must be kept as small as possible for the reasons stated above under (ii) and (v). However, this means that a substantial portion of this window will be covered with an adhesive normally used to mount LED chips and thus will not be able to contribute fullyxe2x80x94if at allxe2x80x94to the coupling-out of the light.
Points (ii) and (v) take on greater significance as the chip edge length decreases, i.e., as the cross section of the active region becomes smaller, an effect that is constantly being striven for in order to obtain the greatest possible chip yield from a single wafer, because the smaller the edge length the smaller the resulting chip mounting surface, in view of the proposed chip geometry. For these reasons, the bottom window layer is made as thin as possible or is omitted.
In terms of practical feasibility, the chip geometry known from DE 198 07 758 A1 is suited, if at all, only to GaP-based material systems, involving the epitaxial growth of thick layers of both types of semiconductors that are sufficiently electrically conductive to make it possible, in particular, to create the proposed lower window layer and simultaneously to achieve current spreading to nearly the entire active region.
In the nitride-based III-V semiconductor material system, which includes in particular GaN, InN and AlN and all ternary and quaternary mixed crystals based on GaN, InN and AlN, such as, for example, AlGaN, InGaN, AlInN and AlGaInN, p-doped layers in particular have a sufficiently low electrical resistance only if they are comparatively thin. Thus, the only way to fabricate a thick bottom window corresponding to the arrangement described abovexe2x80x94especially in the case of conventional GaN-based active layer sequences, in which the bottom window layer would have to be on the p-type sidexe2x80x94is by wafer bonding, which is technically very onerous and means putting up with the above difficulties.
Known from U.S. Pat. No. 5,233,204 is a geometry for a light-emitting semiconductor body based on InGaAlP, in which a thick, transparent epitaxial layer of GaP, GaAsP or AlGaAs is arranged between an absorbing substrate and an active layer structure. The lateral faces of the thick transparent epitaxial layer are arranged obliquely to the active layer structure in such manner as to produce funnel-shaped layer. As a result, more of the radiation emitted by the active layer structure in the direction of the substrate strikes the lateral face of the transparent layer at an angle that is smaller than the total reflection angle.
However, substantial losses of light occur with the chip geometry proposed in U.S. Pat. No. 5,233,204. One cause of these losses is total reflection at the boundary from the active layer structure to the thick transparent epitaxial layer (refractive indexactive layer greater than refractive indexwindow) and subsequent absorption in the active layer sequence. Another cause is absorption in the radiation-absorbing epitaxial substrate. In addition, considerable additional technical outlay is involved in fabricating the window layer as a thick transparent epitaxial layer.
To improve the coupling-out of light, it has been proposed elsewhere to fabricate semiconductor devices having, for example, a triangular or parallelogramoid lateral cross section; in this regard, see the publication Song Jae Lee, Seog Won Song, xe2x80x9cEfficiency improvement in Light-Emitting Diodes Based on Geometrically Deformed Chips,xe2x80x9d SPIE Conference on Light-Emitting Diodes, San Jose, Calif. January 1999, pages 237 to 248. In these arrangements, reflection in the chip is increased because the angles of reflection change frequently. At the same time, however, the radiation-generating layer, the contacts, or other layers of the semiconductor device must be improved so that they absorb as little light as possible.
An object of the invention is to provide a radiation-emitting semiconductor body of the species cited in the introduction hereto that is suitable for the mass production of light-emitting-diode devices and that provides improved coupling-out of light and that in particular ensures a high chip yield from a single wafer. The semiconductor body is intended to be suitable in particular for mounting in conventional light-emitting-diode housings by means of conventional automatic chip mounting systems used in semiconductor technology.
A method for fabricating such a semiconductor body is also provided. A further object of the invention is to provide an improved radiation-emitting optical device.
Hereinafter, the term xe2x80x9cnitride-basedxe2x80x9d encompasses in particular all III-V semiconductor mixed crystals containing binary, ternary and quaternary nitrogen, such as GaN, InN, AlN, AlGaN, InGaN, InAlN and AlInGaN.
Analogously, the term xe2x80x9cSiC-basedxe2x80x9d hereinafter signifies any mixed crystal whose essential properties bear the stamp of the constituents Si and C.
The term xe2x80x9cepitaxial substratexe2x80x9d is to be understood hereinafter as the substrate used as a foundation for the epitaxial growth of the very first layer of the active layer sequence.
Further, the terms xe2x80x9cfront sidexe2x80x9d or xe2x80x9ctop sidexe2x80x9d hereinafter mean that side of the semiconductor body located after the active multilayer sequence in the intended direction of radiation of the device. xe2x80x9cBack sidexe2x80x9d or xe2x80x9cundersidexe2x80x9d therefore means the side facing away from the front side.
In the semiconductor body of the species recited in the introduction hereto, according to the invention the transparent window layer has at least one lateral face which, viewed from the multilayer structure, passes over a first layer-thick section of the window layer obliquely, concavely or in a stepwise manner with respect to a central axis of the semiconductor body lying perpendicular to the plane of growth of the epitaxial layers, and in its subsequent extension in the direction of a mounting surface of the semiconductor body, i.e., over a second layer-thick section adjacent the first layer-thick section, is disposed parallel to the central axis, i.e., vertically to the mounting surface of the semiconductor body.
With reference to the mounting surface of the semiconductor body, the window layer, as its underside, is arranged exclusively beneath the multilayer structure, and the obliquely, concavely or stepwise-extending side walls of the window change over into side-wall portions that extend vertically to the mounting surface and form a parallelepipedoid, prismoid or cylindroid mounting pedestal.
The latter has the particular advantage that the surface area of the underside of the semiconductor device is not greatly diminished, thereby reducing tilting moments of the chip during conventional automated mounting in a housing and thus also decreasing the risk of tilting of the device.
The mounting pedestal further has the advantage that the semiconductor body can be adhesively secured in a housing by this same parallelepipedoid, prismoid or cylindroid bottom portion of the device, which extracts little light. Owing to the geometry according to the invention of the semiconductor body, the surface by which the semiconductor body is affixed to a mounting surface of the housing is large enough to provide, on the one hand, a stable mechanical connection between the semiconductor body and the housing, and on the other hand, a sufficiently large contact area, both electrical and thermal.
A further particular advantage deriving from the parallelepipedoid, prismoid or cylindroid mounting pedestal is that despite the obliquely extending lateral faces, no wedge-shaped gap of the kind normally filled with transparent casting material in a conventional plastic LED housing is created between the semiconductor body and the mounting surface of an appurtenant housing. The forces pushing the semiconductor body away from the mounting surface, brought about by the high thermal expansion of ordinary transparent casting compounds and occurring with an increase in the temperature of the device during operation and/or due to an increase in ambient temperature, as happens to a not insubstantial degree in motor-vehicle applications, for example, are markedly reduced in comparison to known chip geometries comprising oblique window sides. There is consequently much less risk of delamination of the chip from the mounting surface of the housing, which mounting surface also simultaneously serves as an electrical terminal for the semiconductor body when an electrically conductive substrate is used.
The semiconductor body geometry according to the invention further provides the advantage of minimizing the loss of active multilayer structure during the production of this geometry. Nearly all of the wafer area can be used as an active region for the semiconductor body. The number of semiconductor bodies per wafer obtained with the chip geometry according to the invention is unchanged with respect to the fabrication of conventional right-parallelepipedal semiconductor bodies.
A further advantage is that the front side of the device is large enough so that a large portion of the radiation emitted forwardly by the active region can be coupled out directly in the forward direction, without any additional losses, especially absorption losses, in an additional upper window.
The semiconductor body geometry according to the invention is suitable especially preferably for, in particular, semiconductor bodies with a nitride-based active multilayer structure (i.e., of the material system In1xc3x97xxc3x97yAlxGayN, where 0£x£1, 0£y£1 and x+y £1) on an SiC-based substrate or other transparent substrate whose material has a higher refractive index than does the active multilayer structure. One reason for this is that nitride-based layers have a sufficiently low electrical resistance only when they are very thin. This is especially true of p-doped layers of this kind.
Because the overlap between the angular range of the radiation incident on the lateral sides of the substrate and the angular range coupled out by the lateral sides of the substrate is improved with respect to that of conventional right-parallelepipedal chips, the semiconductor body according to the invention is capable of coupling out a comparatively large fraction of the radiation generated in the active region on its very first transit through the chip, i.e., when the radiation strikes the chip surface for the first time. This reduces total reflection on the lateral faces, increases the direct coupling-out of light, and diminishes absorption due to long optical paths and numerous reflections in the device, i.e., in adjacent window regions.
The obliquely disposed, concave or stepped portion of the lateral face is especially preferably realized so that a large portion of the radiation coupled out by the semiconductor device through the lateral sides is emitted within an angular range of between 50xc2x0 and 90xc2x0 relative to the central axis of the semiconductor device. This advantageously achieves the result that in the case of a housing design that includes a reflector, the type of housing conventionally used for light-emitting diode devices, the radiation coupled out laterally through the window layer strikes the oblique side walls of the reflector. This has the significant advantage that the radiation strikes the inside walls of the reflector at a relatively obtuse angle, thereby improving the reflection, especially in the case of plastic reflector walls.
As shown in FIG. 2, in conventional right-parallelepipedal chips, owing to the steep angle of emission, most of the radiation coupled out by the window layer is reflected by the sides of the chip (cf. FIG. 2) onto the floor of the reflector, which reflects comparatively little radiation, on the one hand because of the steeper angle of incidence of the radiation and on the other hand because of the inevitable partial covering of this floor with adhesive.
In a further especially preferred manner, the material of the window layer has a higher refractive index than the adjacent material of the active multilayer structure. As a result, the radiation emitted backwardly by the active region is advantageously reflected less at the boundary between the multilayer structure and the window layer and the radiation coupled into the window layer is compressed.
The chip geometry according to the invention is used especially preferably with nitride-based LED chips whose active multilayer structure is fabricated on an SiC or SiC-based epitaxial substrate. Here, refractive indexactive layer greater than refractive indexsubstrate.
In a chip with a square cross section, the ratio of the edge length of the multilayer structure to the edge length of the mounting pedestal is especially preferably 1.2 to 1.6. In the case of a window with planar, oblique lateral faces, these sides especially preferably form an angle xcex1 with the central axis of the semiconductor device defined by 20xc2x0£xcex1£40xc2x0.
With this embodiment, on the one hand, a good current spread over the surface of the multilayer structure is ensured, and on the other hand, the voltage drop at the mounting pedestal during the operation of the semiconductor device is within an acceptable range.
In a further advantageous embodiment, it is provided that at least the obliquely disposed, concave or stepped region of the window layer is roughened.
It is especially advantageous if the window is the epitaxial growth substrate, such as, for example, in the case of a GaN-based mutlilayer structure on an SiC-based epitaxial substrate forming the window.
In a radiation-emitting optical device according to the invention containing a radiation-emitting semiconductor device having the features disclosed hereinabove, the semiconductor device is mounted in a reflector recess in a housing base body. The reflector recess preferably has an advantageously planar floor on which the radiation-emitting semiconductor device is mounted and which is surrounded by a planar reflector wall obliquely disposed with respect to the bottom. The housing base body is made of a reflective material, especially a suitable plastic, which is advantageously filled with reflection-increasing material, and comprises electrical connecting elements. The semiconductor device is placed with its mounting surface on the floor and connected thereto with adhesive.
The height of the mounting pedestal is so selected that the largest possible proportion of the radiation coupled out by the obliquely disposed, concave or stepped region of the window layer strikes the oblique reflector walls and not the floor of the reflector recess and is reflected in the direction of radiation of the device.
Since the oblique wall surfaces cannot be brought arbitrarily close to the semiconductor devicexe2x80x94it being necessary for the floor to provide enough space, first, for the semiconductor device to be mounted in the housing with a certain positioning tolerance, and second, for a bonding-wire connection to be made from the top side of the chip to an external electrical connecting elementxe2x80x94the mounting pedestal is an essential element of the semiconductor device in accordance with the invention.
In a particular embodiment, the side walls of the reflector are realized as paraboloid in such manner that the radiation coupled out via the obliquely disposed, concave or stepped region of the window layer is reflected upwardly to the active layer, predominantly in parallel, in a given direction.
The lower angle of the radiation relative to the floor carries the further important advantage of reducing absorption of the radiation, especially in the case of reflection on plastic surfaces, because the smaller the angle of incidence of the radiation on the plastic surfaces, the lower the absorption.
The reflector walls can advantageously be rendered highly reflective in a simple manner, for example by coating them with Al or Ag. In addition, there is no risk that the oblique reflector walls will become contaminated with adhesive while the semiconductor device is being mounted in the housing.
The contact metallization on the underside of the mounting pedestal is preferably a gridded contact. It has been found that the back-reflection of backwardly emitted radiation in the direction of the front side of the chip is improved when the contact metallization on the back side is not made to cover its entire surface.
In a method for fabricating a radiation-emitting device in accordance with the invention, it is provided, after the active multilayer structure has been put in place and optionally structured on a large-area substrate wafer, to saw with a saw blade provided with a shaped edge into the wafer so fabricated, from what is to be the back side of the semiconductor device, i.e., from the side of the window layer facing away from the active multilayer structure, to a predetermined depth at which the blade-shaped portion of the saw blade, in which the surfaces of the saw blade are parallel, penetrates partway into the substrate wafer, preferably to the level of the mounting pedestal.
The term xe2x80x9cshaped edgexe2x80x9d is to be understood herein as a working side of the saw blade shaped according to certain criteria to produce a sawcut of a defined geometry. The shaped edge in the present case is the negative shape of the obliquely disposed, concave or stepped portion of the window layer and is therefore configured as V-shaped, convex or stepped.
This step of making a shaped sawcut is followed by the separation of the large-area wafer arrangement, with the multilayer structure thereon, into a plurality of radiation-emitting semiconductor devices by dividing the wafer composite along the sawcuts by breaking it or making a second sawcut.
Especially preferably, the sawing with the saw blade having a shaped edge is preceded by separation of the active multilayer structure along the intended sawcuts, preferably by etching.
In a further preferred manner of implementation of the method, it is provided that the contact surfaces, formed in particular by metallization layers, are produced before the back of the wafer is sawn into.
In an especially preferred improvement of the method, a GaN-based epitaxial layer sequence suitable for emitting radiation in the ultraviolet, blue and/or green regions of the spectrum is first disposed on an SiC-based substrate wafer. The substrate material is preferably so selected that it is at least semitransparent to at least a large proportion of the radiation emitted by the active region of the multilayer structure and has a higher refractive index than the material of the epitaxial layer sequence.
Contact layers are then disposed on the front side of the epitaxial layer sequence and on the back side of the substrate wafer, after which the epitaxial layer sequence is separated, preferably by etching, into a plurality of mutually separate active multilayer structures. This is followed by the step of making a shaped sawcut from the side of the substrate wafer facing away from the epitaxial layer sequence.